This invention relates to fabricating brazeable diamond products and components for the microelectronic industry that are used as heat spreaders, heat conductors and electrical insulators in electronic packages. More particularly, the product of this invention relates to a multilayer metallization structure that allows diamond to be attached into electronic packages using conventional brazing operations. Still more particularly, the products for this invention have applications involving thermal management of high power semiconductor devices by the use of diamond.
Integration of high power electronic devices into electronic systems typically requires the construction of an electronic package. An electronic package is typically comprised of a metal flange onto which is attached an electronic device (xe2x80x9cchipxe2x80x9d). In some package designs, it is important to insure that the chip does not electrically short to the metal flange, a layer of electrically insulating material is positioned between the device and the flange. A window frame or lead frame may be attached around the device. This frame enables electrical connections via wire bonds or other techniques from the chip to the outside of the package. For environmental protection of the chip, a cap attached over the flange may seal the package. The electronic package is then attached to a heat sink, which may be a metal element in contact with a cooling medium such as air, fluorocarbon liquid, and the like.
High power devices such as those used in high speed computing, microwave and RF telecommunications, and the like typically use ceramic materials such as aluminum oxide, beryllium oxide or aluminum nitride as materials. These materials are cut to shape from sheets, or molded to shape, and are readily attached into electronic packages using conventional solders or brazes. As the power electronic devices are miniaturized, and their power output is increased, their operating temperature is dramatically increased. These ceramic materials have relatively low thermal conductivity compared to diamond (aluminum oxide=20 W/mK (watts/meter/xc2x0K), beryllium oxide=260 W/mK, aluminum nitride=170 W/mK). Therefore, they provide a relatively high thermal resistance between the chip and the flange, which ultimately limits the operating power and reliability of the chip.
Because of its properties, Chemical Vapor Deposition (xe2x80x9cCVDxe2x80x9d) diamond is the ultimate electrical insulator for high power device applications. CVD diamond has extremely high electrical resistivity, high breakdown voltage and high thermal conductivity of 1,000 to 2,000 W/mK, up to four times that of copper. Because of its high thermal conductivity, a diamond layer of significant thickness (typically at least 200 microns thick) also functions as a heat spreader, effectively spreading the heat from the localized area at the chip to a larger area at the flange and ultimate heat sink. However, the use of diamond as an electrical insulator in high power electronic devices has been limited because of the cost of manufacturing diamond components, and the inability to reliably attach diamond into device packages using established methods.
Recently, as the manufacturing cost of diamond has declined with improved CVD synthesis techniques, the demand for diamond components has increased. The need for CVD diamond in high power density applications is rapidly increasing as the package sizes decrease and package power increases. Therefore, it is likely that if a robust method for attaching diamond into electronic packages could be developed, diamond heat spreaders and components would be used in almost every application where enhanced thermal management is vital to prolong the life of microelectronic packages. This is especially true for computer chips, associated power supplies and high-frequency telecommunications.
The two most common joining technologies that are used in microelectronic packaging are soldering and brazing. Each of these methods requires metallization of the components and subsequent heating to perform the attachment. Soldering is typically defined as attachment in which requires at temperature of less than 500xc2x0 C. to melt metal layers and join the components, while brazing requires a temperature greater than 500xc2x0 C. to melt the metal layers and join the components.
Solderable metallizations to diamond, such as a multilayer structure of titanium, platinum, gold, followed by gold-tin eutectic solder are well-established. An advantage to the solderable metallizations for diamond is the low attachment temperature, which minimizes thermal stresses resulting from the difference in thermal expansion coefficient between diamond and most materials, especially metals. However, due to the tendency of void formation in soldering, intimate thermal contact may be limited for large areas without specialized bonding techniques.
Brazing is the preferred attachment method for high power electronic packages since the high temperatures involved in the brazing process, typically around 800xc2x0 C. or greater, usually ensure a very good wetting of the braze material at the interface between two components. It is important that there are no voided areas at the component interfaces that are involved in thermal transfer, since any voids would increase the overall thermal resistance of the package.
The interfaces joined by brazing are usually between the insulator, i.e., diamond, and electrical lead frame or flanges. The electrical leads must be able to pass a peel test, and hence the adhesion between the diamond and the leads must be extremely good. Typically, a peel strength of 2.0 pounds minimum at 90xc2x0 is required for leads of 0.15xe2x80x3 in width.
One of the established industrial methods of achieving interface wetting is the use of hydrogen in the atmosphere during the brazing process, since metal oxide formation is prevented. An eutectic alloy, such as Cuxe2x80x94Sil (72% silver, 28% copper), is commonly used for joining articles in brazing processes, but other brazeable materials such as pure copper and gold may be used as well. The Cuxe2x80x94Sil alloy melts at 780xc2x0 C. and wets extremely well to nickel and copper surfaces in an atmosphere of nitrogen containing from 2% up to perhaps 75% hydrogen. Brazing temperatures for Cuxe2x80x94Sil are typically as high as 820xc2x0 C.
To-date, it has been difficult to produce reliable brazeable metallization structures for diamond in microelectronic packages. These difficulties are due to the significant differences in the coefficient of thermal expansion between diamond and metals and also to the chemical nature of the metals comprising the metallization structure.
Many methods are currently known for fabricating diamond products suitable for electronic applications. However, they all have the disadvantage of not being able to withstand brazing temperatures, i.e., temperatures in the range of about 500 to 1100xc2x0 C.
Iacovangelo, et al., U.S. Pat. Nos. 5,324,987 and 5,500,248 describe the use of a diamond product for electronic applications, but which are not suitable for use at such brazing temperatures. One example of the disclosed product is a diamond having an adhesion-promoting material interposed between the diamond and the conductive metal, e.g. copper. The adhesion-promoting material is titanium or a titanium-tungsten alloy. It is believed that the metallization in such a product becomes delaminated when the temperature is increased to brazing temperatures in a standard hydrogen-nitrogen-containing brazing atmosphere because of hydride or nitride formation.
Iacovangelo, et al., U.S. Pat. No. 5,529,805 describe fabrication of brazeable diamond tool inserts using a substantially non-oxidizable nickel protective layer deposited onto a chromium metal layer bonded to the diamond component in a nitrogen-free atmosphere.
Iacovangelo, et al., U.S. Pat. No. 5,567,985 describe an electronic structure comprising a CVD diamond substrate, a tungsten-titanium bond layer, a silver compliant layer, a tungsten-copper layer and a gold solder bond outer layer. Such a structure is capable of surviving a thermal shock of temperature in the range of xe2x88x9265 to 150xc2x0 C. However, such a structure is not capable of surviving brazing in nitrogen-containing atmosphere at temperatures in the range of about 750 to 1100xc2x0 C. without delamination.
From the above discussion it is clear that a diamond product and a method are needed for fabrication of a brazeable diamond component that is extremely robust in terms of adhesion to electrical leads and flanges. There is also a need a diamond component that may be strongly bonded to other package components in a hydrogen-containing and nitrogen-containing brazing atmosphere without delamination. If a hydrogen-containing brazing atmosphere is used, the diamond becomes electrically non-insulating provided that the brazing temperature is sufficiently high.
The product of the present invention includes a diamond component, a chromium metal adhesion layer, a refractory metal barrier layer covering the chromium layer, and a metal wetting layer for joining that covers the refractory metal layer. The diamond component used in the product of this invention is preferably a CVD diamond. Typically, the diamond component is grown in flat sheets to a specified thickness, then separated from the growth substrate. The diamond sheets are polished to the required thickness and surface roughness while the opposing surfaces are kept parallel within a given specification. Typical thickness is in the range of about 100 to 1000 micrometers, preferably about 300 to 500 micrometers. Typical average surface roughness of the diamond surface is 1000 xc3x85 or less. Laser machining can be performed, if necessary, prior to metallization in order to make via holes (xe2x80x9cviasxe2x80x9d) or define the diamond surface dimensions. The deposition of the metal layers is performed sequentially by vacuum deposition methods such as magnetron sputtering, thermal or electron beam evaporation, ion plating or CVD.
The method of the present invention for fabricating a brazeable diamond product comprising the steps of:
(a) depositing the chromium metal adhesion layer onto a surface of the diamond component described above so that the chromium metal layer and the diamond form a strong interface there between and have a thickness of about 150 to about 15,000 xc3x85;
(b) depositing a barrier layer of the refractory metal onto the chromium metal adhesion layer and have a thickness of about 150 to about 15,000 xc3x85;
(c) depositing an outer layer of a metal from the group consisting of copper, silver, gold and mixtures thereof onto the barrier layer; and
(d) recovering a product having at least three layers deposited on the diamond for use in electronic packages.
The chromium metal is first deposited to cover the surface of the diamond that is to be brazed. Preferably the chromium layer has thickness in the range of about 200 to about 10,000 xc3x85. The optimum thickness for complete chromium coverage is at least about 1000 xc3x85. Preferably the refractory metal barrier layer is deposited to a thickness in the range of about 200 to about 10,000 xc3x85. For the barrier layer to achieve optimum performance, and complete coverage of the chromium adhesion layer, the optimum thickness is at least about 1000 xc3x85. The refractory metal barrier layer comprises tungsten, molybdenum, tantalum, niobium, a chromium alloy of the refractory metal, and mixtures thereof. The chromium alloy has about 1 to 20 atomic weight chromium. Tungsten and tungsten-chromium alloys are preferred refractory barrier layers. Finally, the outer or top layer of metal that wets to the braze material, such as copper, silver, gold, is deposited to a thickness from about 200 to 50,000 xc3x85, depending on the particular metal and the brazing temperature. Preferably, the outer layer is copper or silver with a thickness of about 20,000 xc3x85. When the outer layer is gold, preferably the thickness is in the range of about 200 to about 2,000 xc3x85.
The top layer provides a joining layer, contributes to the electrical conduction required in the design of the electronic package, and remains virtually insoluble with the diffusion barrier layer. Because copper, silver and gold are also relatively soft in terms of yield strength, this top layer is somewhat compliant, which lends itself to the robust nature of the metallization when joining materials with a large difference in the coefficient of thermal expansion. The resulting metalized multilayers of the electronic package are easily chemically or sputter etched by conventional photolithographic methods for electrical isolation of devices.
The present invention substantially reduces or eliminates the disadvantages and shortcomings associated with the prior art techniques by providing for the deposition of the multilayers of metals that form a brazeable diamond structure. Optionally after the deposition of the multilayer structure, the coated diamond may be annealed by heating to temperatures of about 850xc2x0 C. for a time such as about 3 minutes. Although this step is unnecessary, it is believed to form an interfacial chromium carbide layer that produces a more robust adhesion and stabilizes the metallization structure for subsequent high temperature processing.
In another embodiment of the method of the present invention, vias in diamond that contain brazeable metallization structures are formed. First, vias in diamond are typically made by laser machining. The shape of vias may have any cross-sectional shape, such as circular or square. The typical vias are cylindrical. The vias may also include the tapering such that the wall of the vias are not perpendicular to the plane of diamond surface, but rather at an angle. The purpose of making the via walls tapered is to increase the amount of material deposited during physical vapor deposition. Tapering the via walls increases the capture cross-section of the vias walls with respect to the material vapor stream during processing in a vacuum chamber operating at less than 1xc3x971031 5 Torr.
Secondly, whereas the purpose of vias are for electrical conduction through the structure, the vias walls are coated with an interfacial layer that provides an adhesion interface between the diamond and any electrically conductive material used for filling the vias that is suitable for the particular application. This is accomplished by first depositing onto the via walls a multilayer metallization structure that is capable of withstanding the aforementioned brazing environment. For example, the multilayer brazeable metallization structure of the present invention can be deposited onto the side of the diamond with tapered vias such that the vias are exposed with their widest diameter towards the deposition flux. A heat treating process is performed to stabilize the bond between of this interfacial coating to the diamond via walls. This is typically done at temperatures in the,range of about 800 to 1000xc2x0 C. in in pure argon or nitrogen, either of which may contain 4% hydrogen, for 2 to 10 minutes.
Thirdly, a conductive material made from any combination of the more conductive family of metals, such as copper, silver, or gold, is placed into the vias by a number of methods. Methods may include, but not limited to, electroplating or screen-printing. These methods are both economical in terms of manufacturing.
In the case of screen-printing, a metal stencil is generally used to print a thick film paste, containing metals, oxides and/or frits, and organic components, into the vias by proper alignment of the apertures in the stencil with the vias. The paste becomes conductive after the organic constituents are removed by firing at high temperatures, usually above 300xc2x0 C. Oxides and/or glass frits in the paste reduce the shrinkage of the paste upon firing due to volatile organic components and reduce the coefficient of the thermal expansion of the conductive material. The thick film pastes are typically fired (sintered) at 800 to 1000xc2x0 C. to become cohesive and to bond to the brazeable metallization structure of the present invention. The process of filling vias by stencil printing and firing may be repeated until the vias in the structure are satisfactorily filled by the conductive component of the paste after firing off the organic components.